regexp_replace question / help needed

Hello,

I am running into a problem and need some pointers on regexp_replace - I
can't seem to find an answer in any of the online resources.

I have a string (like 40,000 with different length and number of
components) of them in a field named "externalurl". I need to replace the
final "/" of the string with "&file=" while preserving the filename and
extension following the "/".

The closest I can get is:

regexp_replace('http://test.com/test/testfile.php','/[^/]*$','&file=')

however this looses the file name and returns:

http://test.com/test&file=

What I am looking for is:

http://test.com/test&file=testfile.php

as a result.

Would anyone here point me in the right direction?

Thanks!
-Chris